Ozone is utilized in a number of industrial processes, including drinking water and waste water treatment and disinfection, pulp bleaching, ozonolysis reactions in fine chemical production, and flue-gas denitrification.
Ozone is unstable decomposing to oxygen under ambient conditions and hence cannot be manufactured, transported and stored in the manner of typical chemicals of commerce. Instead, ozone must be produced at point-of-use at the time it is needed. Since ozone is a toxic material its generation only where and when it is required limits the possibility and potential impact of incidents.
Ozone is typically generated from oxygen utilizing a corona discharge. If oxygen is used as the source of oxygen for a given ozone generator then ozone concentrations of 10 to 15% by weight (balance oxygen) can be prepared. If air is used as the source of oxygen then ozone concentrations of 1.5 to 3% (balance air) can be prepared. For moderate to large ozone requirements, the overall capital plus operating costs are typically less when oxygen is used as the oxygen source.
Ozone is therefore often utilized at 10 wt %, ozone with the balance being largely oxygen. It has long been recognized that the re-use of the oxygen from the ozone/oxygen mixture generated by oxygen-based ozone generators would substantially improve the economics for ozone generation. Many schemes have been proposed for the separation of ozone from the ozone/oxygen output stream from the ozone generator, thus allowing oxygen to be recycled back to the ozone generator. In most of these schemes the separated ozone is displaced into the final ozone-utilizing process by means of an inert gas stream. Balcar et al. Ozone Chemistry and Technology, pp 53 to 59, Advances in Chemistry; American Chemical Society, Washington, D.C. 1959 proposed the cryogenic liquefaction of the ozone, with re-evaporation of the condensed ozone into a carrier gas. Kiffer et al (U.S. Pat. No. 2,872,397) and Cook et al., Ozone Chemistry and Technology, pp. 44 to 52, Advances in Chemistry; American Chemical Society, Washington, D.C. 1959 proposed the use of a silica gel adsorbent to selectively adsorb ozone from the ozone/oxygen mixture after cooling of the stream, with subsequent desorption of the ozone to the customer process by means of an inert gas, such as air, nitrogen, argon, etc., or by application of a vacuum.
Many improvements and variations on the use of selective adsorbents to allow recycle of the unused oxygen to the ozone generator and an ozone product stream have been proposed. These include the following patent specifications.
U.S. Pat. No. 4,786,489 targets reducing the costs for large scale ozone and teaches the use of a low temperature (−80 to −90° C.) ozone/oxygen separation unit that is purged with an impure nitrogen carrier gas stream containing oxygen and/or air to give the ozone product.
U.S. Pat. No. 5,520,887 is targeted at reducing the costs of ozone generation for pulp bleaching and teaches the use of a PSA oxygen generator to provide an enriched oxygen feed to an ozone generator, an oxygen ozone PSA to adsorb ozone and at the same time recycle oxygen to the ozone generator. The nitrogen rich waste gas from the O2 PSA is used to purge adsorbed ozone from the ozone oxygen PSA to the ozone consuming process.
U.S. Pat. No. 6,030,598 describes the production of an ozone containing gas stream by subjecting oxygen to an electric discharge, adsorbing the ozone thus generated on to a solid adsorbent (such as zeolite) and recycling the oxygen containing stream leaving the adsorbent to the ozonising process. Periodically, oxygen adsorbed on the adsorbent is desorbed by co-currently passing a purge gas over the adsorbent and the desorbed oxygen is also recycled to the ozonizer. Ozone is desorbed from the adsorbent by a counter-current flow of purge gas and used in the ozone demanding process. A 3-bed (or multiple thereof) process and cycle is described that allows ozone and recycled oxygen to be produced continuously, but still requires each bed to experience a non-productive hold step within a full cycle.
U.S. Pat. No. 6,197,091 describes the use of an ozone/oxygen membrane separation system in which ozone permeates through the membrane and is carried with a carrier gas, such as nitrogen, argon or CO2 into the ozone utilizing application, and at the same time the oxygen enriched stream is recycled to the ozone generator.
U.S. Pat. No. 6,916,359 describes a method of providing ozone at a pressure above atmospheric pressure that comprises an ozone generator and an oxygen ozone PSA system. The unadsorbed oxygen from the PSA is recycled back to the ozone generator and the ozone product is carried into the ozone application by means of an inert gas stream at a pressure such that no further compression is needed. The carrier gas can be nitrogen, but is preferentially compressed air used also to feed a PSA oxygen generator that can be used as the oxygen source.
U.S. Pat. No. 7,766,995 is targeted at reducing the cost of ozone utilized in the removal and capture of NOx from industrial flue-gas and other process streams. It teaches the use of an oxygen ozone separation means to allow recycle of oxygen back to the ozone generator and the use of clean dry air to carry the ozone into the industrial process. Optimum ozone production costs are achieved by recycling oxygen to the ozone generator, using the cheapest possible carrier gas to carry the ozone to the point of use, and reducing the power utilization in the ozone generator by operating the generator at lower ozone concentrations than normal (e.g., 6%).
Many attempts have been made to develop a process to reduce the cost of ozone generation from oxygen by recovering the un-utilized oxygen and recycling this stream back to the ozone generation, but with limited commercial application.
What is needed is an efficient and reliable process that overcomes the fluctuations in pressure and concentration typically found in the products of PSA separation processes, without the need for large and expensive buffer tanks and which prevents the build-up of weakly adsorbed species, such as N2 and Ar, in the recycled gas stream. The process of this invention provides the solution to these problems.